1. Field of the Invention
This invention is directed to a mask and method for laser sealing a glass envelopes, such as frit sealed flat OLED display panels.
2. Technical Background
Organic light emitting diodes (OLEDs) have been the subject of considerable research in recent years because of their use and potential use in a wide variety of electroluminescent devices. For instance, a single OLED can be used in a discrete light emitting device or an array of OLEDs can be used in lighting or flat-panel display applications (e.g., OLED displays). OLED flat panel displays in particular are known to be very bright and to have good color contrast and wide viewing angle. However, OLED displays, and in particular the electrodes and organic layers located therein, are susceptible to degradation resulting from interaction with oxygen and moisture leaking into the OLED display from the ambient environment. It is well known that the life of the environmentally sensitive OLED display can be significantly increased if the electrodes and organic layers located therein are hermetically sealed from the ambient environment. Unfortunately, in the past it has been very difficult to develop a sealing process to hermetically seal the OLED display. Hermetically sealing an environmentally sensitive device such as an OLED is complicated by the fact that the OLED material is sensitive to temperature and will be destroyed if heated to temperatures exceeding about 100 to about 120 degrees Celsius.
One way to seal the temperature sensitive OLED devices located between a cover glass and a substrate glass is to place a line of low temperature glass frit, doped with a material that is highly absorbent at a specific wavelength of light, between the cover glass and the substrate glass and surrounding the OLED devices. A hermetic seal is formed around the OLED device by melting the frit with, for example, a high power laser or other radiation source, thereby heating and melting the frit. As the flit line cools it hardens, joining the cover glass to the substrate glass and forming a hermetic seal around the OLED devices. Use of a high power laser enables quick localized heating of the frit without overheating the temperature sensitive OLED material that is closely adjacent the line or wall of frit surrounding the OLED device. The line or wall of frit is typically about 0.5 mm to 1 mm wide and approximately 6-100 um thick or high, but in some instances may be about 1.5 mm or more wide.
It is known to frit seal OLED devices through irradiation with a laser beam having a Gaussian profile. Due to the profile of the laser beam, the thermal distribution created transversely across the frit line is a gradient, with a peak temperature at the center of the frit line. The peak temperature in the frit during laser sealing may be as high as approximately 600° C. This peak temperature can result in undesirable changes in materials located between the frit and the backplane, such as melting lead materials, causing separation of lead materials from the backplane, or any other number of thermally induced changes in backplane materials. There is a need in the art for an effective, high yield (e.g. low defective seal rate) laser frit sealing process for hermetically sealing environmentally sensitive devices (e.g. devices that are sensitive to oxygen, moisture, or other elements in the ambient environment) that are also sensitive to elevated temperatures, such as OLEDs, while avoiding thermal damage to the devices located within the flit wall and to other backplane material located between the frit and the backplane.